staging: bcm2835-codec: Allow decode res changed before STREAMON(CAPTURE)

The V4L2 stateful video decoder API requires that you can STREAMON
on only the OUTPUT queue, feed in buffers, and wait for the
SOURCE_CHANGE event.
This requires that we enable the MMAL output port at the same time
as the input port, because the output port is the one that creates
the SOURCE_CHANGED event.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>